CuO-doped NaNbO(3) antiferroelectrics: Impact of aliovalent doping and nonstoichiometry on the defect structure and formation of secondary phases

The interplay between aliovalent CuO doping and nonstoichiometry on the development of defect structures and the formation of secondary phases of antiferroelectric NaNbO(3) ceramics has been investigated by means of x-ray diffraction (XRD), first-principles calculations using density functional theory (DFT), and electron paramagnetic resonance spectroscopy. The results indicate that, for stoichiometric 0.25 mol% CuO-doped NaNbO(3), as well as for 2.0 mol% Nb-excess sodium niobate, the Cu(2+) functional centers are incorporated at the Nb site (Cu'''(Nb)). For reasons of charge compensation, two kinds of mutually compensating defect complexes (Cu'''(Nb) - V(O)(center dot center dot))' and (V(O)(center dot center dot) - Cu(Nb)(center dot center dot) - V(O)(center dot center dot))(center dot) are formed where, for the niobium-excess compound, additionally, V'(Na) contribute to the mechanism of charge compensation. In contrast, for 2.0 mol% Na-excess sodium niobate, a Na(3)NbO(4) secondary phase has been detected by XRD, and only part of the Cu(2+) forms these types of defect complexes. The major part of the Cu(2+) is incorporated in a fundamentally different way by forming Cu(2+)-Cu(2+) dimeric defect complexes